Organic π-conjugated oligomers and polymers constitute a class of promising semiconducting materials having demonstrated utility in device applications ranging from light-emitting diodes [Segura, J. L. Acta Polym. 1998, 49, 319-344; Mitschke, U.; Bäuerle, P. J. Mater. Chem. 2000, 10, 1471-1507]; photovoltaic cells [Yu, G.; Gao, J.; Hummelen, J. C; Wudl. F.; Heeger, A. J. Science 1995, 270, 1789-1791; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 15-26], field-effect transistors [Katz, H. E. J. Mater. Chem. 1997, 7, 369-376; Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359-369] to nonlinear optics [Nalwa, H. S. Adv. Mater. 1993, 5, 341-358; Tykwinski, R. R.; Gubler, U.; Martin, R. E.; Diederich, F.; Bosshard, C; Günter, P. J. Phys. Chem. B 1998, 102, 4451-4465]. Reducing and tuning energy gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of such π-conjugated species play crucial roles in optimizing the performance of electronic and optical devices based on active organic components [Tour, J. M. Chem. Rev. 1996, 96, 537-553; Roncali, J. Chem. Rev. 1997, 97, 173-205; van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Meijer, E. W. Chem. Eur. J. 1998, 4, 1235-1243; Martin, R. E.; Diederich, F. Angew. Chem. Int. Ed. 1999, 38, 1350-1377; van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Having a, E. E.; Meijer, E. W. Mater. Sci. Eng. 2001, 32, 1-40; Ajayaghosh, A. Chem. Soc. Rev. 2003, 32, 181-191; Sonmez, G.; Meng, H.; Wudl, F. Chem. Mater. 2003, 15, 4923-4929; Chen, M.; Perzon, E.; Andersson, M. R.; Marcinkevicius, S.; Jönsson, S. K. M.; Fahlman, M.; Berggren, M. Appl. Phys. Lett. 2004, 84, 3570-3572].
Porphyrins are tetrapyrrolic conjugated macrocyclic systems that possess modest potentiometrically determined HOMO-LUMO gaps (Ep; E1/20/+-E1/2−/0) relative to those of the common monomeric aromatic building blocks used to construct traditional electronic polymers. The electronic properties of (porphinato)metal compounds can be modulated extensively by variation of the macrocycle peripheral meso- or β-substituents, as well as by selection of the central metal ion; further, a variety of modes of porphyrinoid-porphyrinoid connectivity provides sufficiently strong interchromophore electronic interactions to facilitate extensive electronic derealization. Of these families of multipigment ensembles that feature substantial ground- and excited-state interchromophore electronic interactions, those that feature direct ethyne-, butadiyne-, and oligoyne-based macrocycle-to-macrocycle connectivity have evinced a wide range of particularly impressive electrooptic properties. As increasing conjugation length diminishes significantly optical (Eop) and potentiometric (Ep) band gaps within these families of structures, multiporphyrin compounds that exploit cylindrically π-symmetric linkers define a point of reference from which to engineer further electronic modulation of conjugated organic materials.
An established means to further reduce the Eop and Ep gaps of π-conjugated materials involves introducing quinoid-like character into the conjugation main-chain [Roncali, J. Chem. Rev. 1997, 97, 173-205; van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Having a, E. E.; Meijer, E. W. Mater. Sci. Eng. 2001, 32, 1-40; Ajayaghosh, A. Chem. Soc. Rev. 2003, 32, 181-191], Solution-phase spectroscopic experiments [Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264, 1105-1111; Lin, V. S.-Y.; Therien, M. J. Chem. Eur. J. 1995, 1, 645-651; Susumu, K.; Therien, M. J. J. Am. Chem. Soc. 2002, 124, 8550-8552; LeCours, S. M.; DiMagno, S. G.; Therien, M. J. J. Am. Chem. Soc. 1996, 118, 11854-11864] and X-ray crystallographic data [Uyeda, H. T. Doctoral Dissertation, University of Pennsylvania, Philadelphia, 2002] obtained for bis[(5,5′,-10,20-di(aryl)porphinato)zinc(II)]ethyne compounds demonstrate that the bridging ethyne possesses conventional triple bond character in the ground state; electronic absorption [Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264, 1105-1111; Lin, V. S.-Y.; Therien, M. J. Chem. Eur. J. 1995, 1, 645-651, Shediac, R.; Gray, M. H. B.; Uyeda, H. T.; Johnson, R. C; Hupp, J. T.; Angiolillo, P. J.; Therien, M. J. J. Am. Chem. Soc. 2000, 122, 7017-7033; Susumu, K.; Therien, M. I. J. Am. Chem. Soc. 2002, 124, 8550-8552], electroabsorption [Shediac, R.; Gray, M. H. B.; Uyeda, H. T.; Johnson, R. C; Hupp, J. T.; Angiolillo, P. J.; Therien, M. J. J. Am. Chem. Soc. 2000, 122, 7017-7033] and pump-probe spectroscopic methods [Kumble, R.; Palese, S.; Lin, V. S.-Y.; Therien, M. J.; Hochstrasser, R. M. J. Am. Chem. Soc. 1998, 120, 11489-11498; Rubtsov, I. V.; Susumu, K.; Rubtsov, G. I.; Therien, M. J. J. Am. Chem. Soc. 2003, 125, 2687-2696] are consistent with an excited state electronic structure for this species that features a modest degree of cumulenic (quinoidal) character. Porphyrin-to-porphyrin bridging motifs involving ethynes and spacers that induce a quinoidal structural perturbation with appropriately positioned frontier orbital energy levels, should enhance ground- and excited-state π-conjugation, and effect further reduction in Eop and Ep in the corresponding oligomeric and polymeric structures
Polymer band-gap reduction through augmentation of π-backbone quinoidal character has been explored both experimentally [Kobayashi, M.; Colaneri, N.; Boysel, M.; Wudl, F.; Heeger, A. J. J. Chem. Phys. 1985, 82, 5717-5723; Jenekhe, S. A. Nature 1986, 322, 345-347] and theoretically [Brédas, J. L.; Heeger, A. J.; Wudl, F. J. Chem. Phys. 1986, 85, 4673-4678; Lee, Y.-S.; Kertesz, M. J. Chem. Phys. 1988, 88, 2609-2617]. In this regard, benzo[1,2-c:4,5-c′]bis([1,2,5]thiadiazole) (BBTD), exemplifies an established conjugated unit with suitable electronic structure to induce substantial quinoidal character in a conjugated backbone [Ono, K.; Tanaka, S.; Yamashita, Y. Angew. Chem. Int. Ed. Engl. 1994, 33, 1977-1979; Karikomi, M.; Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Am. Chem. Soc. 1995, 117, 6791-6792; Kitamura, C; Tanaka, S.; Yamashita, Y. Chem. Mater. 1996, 8, 570-578; Yamashita, Y.; Ono, K.; Tomura, M.; Tanaka, S. Tetrahedron 1991, 53, 10169-10178].
Near infrared emissive fluorophores (NIRFs) have found many uses, including optical imaging. As their utilization increases, new NIRFs are needed.